Coarse Aggregate Particles Research Articles

Evaluating the Homogeneity of the Interval Quality of Asphalt Pavement Structure with X-Ray Computed Tomography

AbstractThe method that uses only a compaction degree indicator in the core specimens of asphalt pavement for evaluating construction quality cannot effectively reflect the volumetric property and inner structure of asphalt mixtures. Virtually reconstructed specimens are generated by cone-beam scanning field cores with a high-accuracy industrial X-ray computed tomography, and the vertical slice images and digital processing of the aggregate are adopted. To evaluate the vertical structural segregation of the field core specimens, indicators such as the non-uniformity coefficient of coarse aggregate and the vertical variation coefficient of specimens are presented. Results indicate that (1) in various construction control situations, the inner structures of asphalt pavement with the same graded mixture have obvious differences; (2) coarse aggregate particles in the specimens show the regularity of homogeneity in the middle and segregation at both bottoms; and (3) pavement specimens have a gentle homogeneity...

Bond strength prediction for deformed steel rebar embedded in recycled coarse aggregate concrete

Abstract This paper summarizes the results of an experimental investigation into the bond behavior between recycled aggregate concrete (RAC) and deformed steel rebars, with the main variables being the recycled coarse aggregate replacement ratio (RCAr) and water-to-cement ratio of the concrete mixture. The investigation into splitting cracking strength indicates that the degradation of the bond splitting tensile stress of the cover concrete was affected by not only the roundness of the coarse aggregate particles but also the weak interfacial transition zone (ITZ) between the cement paste and the RCA that has a more porous structure in the ITZ than normal concrete. In this study, a linear relationship between the bond strength and the density of the RCA was found, but the high compressive strength reduced the effects of the parameters. To predict the bond strength of RAC using the main parameters, a multivariable model was developed using nonlinear regression analysis. It can be inferred from this study that the degradation characteristic of the bond strength of RAC can be predicted well, whereas other empirical equations and code provisions are very conservative.

Influence of aggregate particles on mastic and air-voids in asphalt concrete

The main objective in this paper is to determine the effect of aggregate particles on internal structure in asphalt concrete. Binary images of real microstructures were obtained based on Otsu thresholding method with different annular regions and three-dimensional finite element models were reconstructed. Prony series inferred from Generalized Maxwell Model were used to describe the viscoelastic behavior of asphalt mastic, and then, using numerical simulations to investigate the effect of aggregate particles on stress and strain field in asphalt mastic at high temperature. In addition, Box-counting method calculated the fractal dimensions of aggregate particles to analyze the influences of aggregate shape on air-voids. Results indicate that the improved Otsu can effectively extract the microstructural characteristics of aggregate particles, enhancing the accuracy of numerical simulation and calculation; The proportion of coarse aggregates and corresponding sizes have an obvious impact on stress field in asphalt mastic, high-temperature damage is mainly caused by coarse aggregate particles; The major component of total strain in asphalt mastic is creep strain, and the creep strain increases with the increase of aggregate particle size; fractal dimensions of aggregate particles have significant effect on air-voids distribution.

Adding ground sand to decrease paste volume, increase cohesiveness and improve passing ability of SCC

In the production of SCC (self-consolidating concrete), it is not easy to achieve high passing ability without sacrificing other performance attributes because the common practice of increasing the cementitious paste volume to achieve high passing ability would decrease the dimensional stability and increase the carbon footprint. In fact, at narrow gaps, the paste often flows in the front leaving the coarse aggregate particles behind to block the subsequent flow of concrete, especially if the paste does not have good cohesiveness. Hence, increasing the paste volume is not always effective in increasing the passing ability. Herein, it is proposed to keep the paste volume small and add ground sand with a mean size of 0.3mm to increase the mortar volume and cohesiveness, and improve the passing ability. Trial concrete mixes with paste volumes of 30% and 34% and different ground sand contents have been tested and the results demonstrated that the addition of ground sand would enable the achievement of high passing ability at a relatively small paste volume.

Mix Proportion Design and Mechanical Properties of Recycled PET Concrete

Abstract The mix proportion and mechanical properties of polyethylene terephthalate (PET) concrete were studied by orthogonal test. PET concrete was produced by mixing melted PET with aggregate and mineral admixtures. The effects of several factors on the properties of PET concrete were investigated. These factors included: (1) coarse aggregate type, (2) mineral materials, (3) PET and mineral/aggregate (PETAM/AG) ratio, (4) sand ratio, (5) PET/mineral (PET/MA) ratio, and (6) maximum particle size of coarse aggregate. The results showed that the most important factor was PETAM/AG, followed by sand ratio and PET/MA. Samples with 10 mm coarse aggregate had the highest compressive strength of 36 MPa. The compressive strength decreased 8.6 and 37 % as the coarse aggregate maximum particle sizes changed from 10 to 4.75 and 13 mm, respectively. There was no significant difference in flexural strength when the sand ratio changed from 35 to 68 %. The strength of PET concrete with basalt was higher than that of concrete with limestone. The concrete containing fly ash and 5 % talc had the highest strength followed by concrete with fly ash, Portland cement (PC), fly ash with 5 % asphalt, without any mineral. The best mix proportion of PET concrete was PETAM/AG of 1:3, sand ratio of 35 % and PET/fly ash of 2:1.

Estimation of the yield stress and distribution of large aggregates from slump flow test of self-compacting concrete mixes using smooth particle hydrodynamics simulation

A wide range of normal strength self-compacting concrete (SCC) mixes with a maximum aggregate size (g) of 20 mm ranging in compressive strength from 30 to 80 MPa were prepared in the laboratory, and the times t500 and tstop of each mix were recorded in the slump flow test. The entire cone flow test was then simulated from the moment the cone was lifted until the mix stopped to flow using the three-dimensional mesh-less smoothed particle hydrodynamics computational approach, treating the SCC mix as a non-Newtonian Bingham fluid. This numerical simulation had two aims. First, to investigate whether the yield stress τy of the mix could be accurately estimated from the measured t500 and tstop times knowing the plastic viscosity of the mix. The latter was estimated by a micromechanical procedure. Second, to compare the distribution of coarse aggregate particles larger than or equal to 8 mm in the cone spread after it stopped to flow as revealed by the numerical simulation with the distribution of the aggregate particles of the corresponding sizes in the cut sections of the cured test cone spread. The large coarse aggregate particles in the size ranges (8 ≤ g < 12, 12 ≤ g < 16, 16 ≤ g < 20 and g ≥ 20 mm) of the test SCC mixes were color coded with non-toxic non-water soluble paints so that the outlines of the aggregate particles could be clearly distinguished in the cut sections of the hardened cone spread and compared with the numerical simulations. It is shown that both aims of the investigations were successfully attained.

Chloride ingress in cracked concrete: a laser induced breakdown spectroscopy (LIBS) study

racks are always present in reinforced concrete structures. In the presented research, influence of mechanical cracks on chloride ingress is studied. A compact reinforced concrete specimen was designed, mimicking the cracking behaviour of beam elements. Cracks of different widths were induced by means of mechanical loading. These cracked specimens were then subjected to weekly cycles of wetting and drying with NaCl solution. After the exposure, the specimens were cut, and chloride distributions were determined using Laser Induced Breakdown Spectroscopy (LIBS), an innovative technique which enables simultaneous determination of different elements with high spatial resolution and minimal specimen preparation. By combining element distributions of different elements, it is possible to discriminate between coarse aggregate particles, and the mortar matrix. It was found that the wider the crack is, the higher the ingress of chloride ions. This was, however, different for two tested concrete mixes. Due to highly inhomogeneous chloride distribution around the cracks, use of fine-scale experimental techniques for chloride mapping is advised, based on the presented study.

Effect of particle interlock on flow of aggregate through opening

Blockage of concrete flow through opening or gap often occurs during placement of self-consolidating concrete due to particle interlock of coarse aggregate particles. Suggestions to reduce the coarse aggregate content in order to minimize such particle interlock have been made by some researchers. However, reduction of the coarse aggregate content would also decrease the particle packing density and, if overdone, might adversely affect the overall performance of the concrete. A better understanding of how particle interlock would occur is desperately needed. Herein, a systematic study on particle interlock and flow of aggregate through opening is reported. It will be shown that the aggregate grading has great effects on the flow of aggregate through opening and that to alleviate particle interlock, the fine aggregate content should be more than sufficient to fill the voids between the coarse aggregate particles so as to provide a certain thickness of excess fine coating the coarse aggregate particles.

MESOSCALE MODELING OF CONCRETE UNDER DYNAMIC SPLIT TENSION

In this paper, a mesoscale model is adopted to simulate concrete behavior under dynamic split tension. The concrete material is assumed to comprise coarse aggregates, mortar matrix, and an interfacial transition zone (ITZ). In the mesh generation process, random coarse aggregate particles are generated from a certain aggregate size distribution and then placed into the mortar matrix with ITZ between the coarse aggregate edge and the mortar matrix. Different aggregate shapes, such as circular, oval, and polygons are modeled to analyze the gravel and crushed stone aggregates. Numerical simulation is used to model the dynamic damage responses of a typical cylinder concrete specimen and a cube specimen under split tension. Velocity boundary is added as the dynamic loading. Reasonable tensile stress–strain relationships are obtained at the macroscale level; and the detailed stress wave distribution and the crack pattern are obtained at the mesoscale level. These numerical results agree well with conventional experimental results. It also shows that cracks are affected by aggregate distribution.

Effects of Compactor Types on Aggregate Orientation of Asphalt Mixtures

More than 90% of Malaysian roads are constructed using asphaltic concrete. However, p revious investigations of a sphaltic concrete have mainly concentrated on the macroscopic properties of the composite materials based on the assumption that the mixtures are homogeneous and isotropic. This paper applies a digital image processing technique to co mpare the orientations of coarse aggregate particles in asphaltic concrete compacted using Marshall, Servop ac and Presbox compactors. Aggregate orientation was measured in terms of a vector magnitude and the average major axis angle. The average major axis angle of all compactors are less than 45° indicating that aggregate orientation in all of the prepared specimens have the preferential orientation along the horizontal plane. The vector magnitude results of the Presbox compacted sample is the least, indicating the presence of the most randomly oriented aggregate. However, the Servopac specimens have more randomly oriented aggregate compared to Marshall specimens. An ANOVA analysis for vector magnitude indicates that only compaction method, not Geometrically Cubical Shaped (GCS) proportion, has a s ignificant impact on aggregate orientation randomness.

Simulation Study on the Stress Distribution in Modeled Recycled Aggregate Concrete under Uniaxial Compression

To investigate the stress distribution in recycled aggregate concrete (RAC) under uniaxial compression, modeled recycled aggregate concrete (MRAC) was studied by numerical simulation. The mechanical properties of interfacial transition zones (ITZs) of RAC were measured by the nanoindentation technique. A two-dimensional numerical study of the stress distribution characteristics of MRAC under the uniaxial compression is presented. The simulation was verified by experimental results. A parametric analysis is then conducted to study the sensitivity of each phase’s mechanical properties and the amounts of old cement mortar in the MRAC. Simulation results demonstrate that a concentration of tensile stress and shear stress appears around new and old ITZ regions. It is found that when the elastic modulus of natural aggregates increases, the magnitude of tensile stress concentration becomes higher, whereas as the elastic modulus of ITZs increases, the magnitude of stress concentration decreases. It is also shown that the higher relative elastic modulus of new cement mortar compared with that of the old cement mortar significantly reduces the stress concentrations at the regions between recycled coarse aggregate particles. The amount of old cement mortar affects the stress distribution in the new ITZ much more obviously than that in the old ITZ.

Statistical modelling of the coupled effect of mix design and rebar spacing on restricted flow characteristics of SCC

Self-consolidating concrete (SCC) is designed to exhibit high deformability and stability to maintain homogenous suspension of coarse aggregate particles in the mortar during flow to ensure adequate passing ability of the concrete through restricted areas and the filling of the formwork without any consolidation. Proper deformability and uniform suspension of coarse aggregate particles in the matrix are key parameters to ensure successful casting and adequate in situ performance. The required flow properties of concrete are affected by the rheological properties of the mixture and the characteristics of the cast element, including the level of reinforcement.The mixture proportioning of SCC involves tailoring several mixture parameters to achieve adequate stability and reduce the risk of arching of solid particles and blockage at the vicinity of closely spaced obstacles which is required to ensure proper passing ability and flow characteristics. The objective of this study is to evaluate the coupled effect of mixture parameters and density of reinforcement on flow characteristics of SCC using a statistical design approach. This study seeks to model the coupled effect of the content of supplementary cementitious materials (SCMs), coarse aggregate volume (CA), and density of reinforcement on flowability, passing ability, and filling capacity of SCC. The comparison between predicted and measured responses determined on eight mixtures indicates the accuracy of the established models to describe the effect of the independent variables on the flow behavior of SCC through restricted sections. The derived models show that the flow rate of SCC in the presence of reinforcement is dominated by the content of SCM and clear spacing between the reinforcement. On the other hand, the SCM content is shown to have a three-time greater effect on plastic viscosity than the relative volume of coarse aggregate. Trade-offs between rheological parameters, clear spacing, and flow of SCC in the presence of obstacles showed that for a given clear spacing, higher viscosity and lower yield stress values resulted in higher flow time of concrete. For clear spacing between obstacles of 30mm, SCC mixtures with plastic viscosity lower than 50Pas and yield stress of 45Pa are shown to ensure proper passing ability.

경량골재 콘크리트의 압축강도에 대한 시험체 기하학적 특성의 영향

Dept. of Architectural Engineering, Kyonggi University, Suwon 443-760, KoreaABSTRACT The current study prepared 9 laboratorial concrete mixes and 3 ready-mixed concrete batches to examine the sizeand shape effects in compression failure of lightweight aggregate concrete (LWC). The concrete mixes were classified into threegroups: normal-weight, all-lightweight and sand-lightweight concrete groups. For each concrete mix, the aspect ratio of circularor square specimens was 1.0 and 2.0. The lateral dimension of specimens varied between 50 and 150 mm for each laboratorial con-crete mix, whereas it ranged from 50 to 400 mm with an incremental variation of 50 mm for each ready-mixed concrete batch. Testobservations revealed that the crack propagation and width of the localized failure zone developed in lightweight concrete spec-imens were considerably different than those of normal-weight concrete (NWC). In LWC specimens, the cracks mainly passedthrough the coarse aggregate particles and the crack distribution performance was very poor. As a result, a stronger size effect wasdeveloped in LWC than in NWC. Especially, this trend was more notable in specimens with aspect ratio of 2.0 than in specimenswith that of 1.0. The prediction model derived by Kim et al. overestimated the size effect of LWC when lateral dimension of spec-imen is above 150 mm. On the other hand, the modification factors specified in ASTM and CEB-FIP provisions, which are usedto compensate for the shape effect of specimen on compressive strength, were still conservative in LWC.Keywords : size effect, compressive strength, lightweight aggregate concrete, shape of specimen

Lattice Discrete Particle Model for Fiber-Reinforced Concrete. I: Theory

The lattice discrete particle model (LDPM) is a mesoscale model for heterogeneous materials. Developed for concrete, it simulates material mesostructure by modeling coarse aggregate particles and their surrounding mortar as polyhedral cells. A tetrahedralization of the particle centers generates a lattice framework where each lattice member is associated with a triangular-shaped plane of contact (facet) between two cells. Compatibility equations are formulated by describing the deformation of an assemblage of particles through rigid-body kinematics. Equilibrium equations are obtained through the force and moment equilibrium of each cell. The material behavior is assumed to be governed by a vectorial constitutive law imposed at the facets. A natural extension for this discrete model is to include the effect of dispersed fibers as discrete entities within the mesostructure. The LDPM incorporates this effect by modeling individual fibers randomly placed within the framework according to a given fiber volume fraction. The number and orientation of the fibers crossing each facet is computed and the contribution of each fiber to the facet response is formulated on the basis of a previously established micromechanical model for fiber-matrix interaction. The theory for the developed model, entitled the LDPM-F, is discussed herein. A subsequent companion paper will address model calibration and validation through the numerical simulation of experimental test results.

Study on Coarse Aggregate Gradation Analysis of Cement Treated Base Course Based on Core Sample Image

In order to get the coarse aggregate particle gradation of cement treated base course core sample dilled form the pavement, an image acquisition system is designed. The system mainly contains an Aviiva ® M2 CL line scan camera, a core sample rotation platform and a LED light source. The software are designed and implemented by using the Matrox Mil controls in the Visual Studio VB.NET. Test results show that when the camera exposure time is set to 800μs, good quality image of core sample can be obtained. In the study, the original core sample images of cement treated aggregate were improved by rescaling of each pixel, then were binarized by using adaptive threshold method. After getting the binary image, morphological operations were carried out to deal with the conglutinations of the particles. Next, characteristics of each particle, such as area, perimeter, fitting ellipse and minFeret were extracted to calculate the area passing rate of aggregate with different size. Results shows that, in this way, the gradation of coarse aggregate (&gt;2.36mm) can be got and the method helps to get the coarse aggregate particle distribution of cement treated aggregate core sample quickly.

Microstructure-Based Inherent Anisotropy of Asphalt Mixtures

Asphalt mixtures have been demonstrated to be anisotropic materials in both laboratory and field tests. The anisotropy of asphalt mixtures consists of inherent anisotropy and stress-induced anisotropy. In previous work, the inherent anisotropy of asphalt mixtures was quantified by using only the inclination angles of the coarse aggregate particles in the asphalt mixtures. However, the inclination of fine aggregates also has a contribution to the inherent anisotropy. Moreover, the contribution to the inherent anisotropy of each aggregate may not be the same as in the previous work but will depend on the size, orientation, and sphericity of the aggregate particle. This paper quantifies the internal microstructure of the aggregates in asphalt mixtures by using an aggregate-related geometric parameter, the vector magnitude. The original formulation of the vector magnitude, which addresses only the orientation of coarse aggregates, is modified to account for not only the coarse aggregate orientation, but also the size, orientation, and sphericity of coarse and fine aggregates. This formulation is applied to cylindrical lab-mixed lab-compacted asphalt mixture specimens varying in asphalt binder type, air void content, and aging period. The vertical modulus and the horizontal modulus are also measured by using nondestructive tests. A relationship between the modified vector magnitude and the modulus ratio of the vertical modulus to the horizontal modulus is developed to quantify the influence of the inherent microstructure of the aggregates on the anisotropy of the mixtures. The modulus ratio is found to depend solely on the aggregate characteristics including the inclination angle, size, and sphericity, and it is independent of the asphalt binder type, air void content, and aging period. The inclination angle, itself, proves to be insufficient to quantify the inherent anisotropy of the asphalt mixtures.

Properties of Lightweight Concrete Masonry Units Made from Lightweight Aggregates

In the paper the properties of concrete masonry unit (CMU) made from sedimentary lightweight aggregate (LWA) were investigated. The main variables include water to cementitious material ratio (W/CM), filling ratio of paste or mortar in voids between coarse aggregate particles (Fv), filling ratio of sand in mortar (Fm), and cement replacement level by slag (Sc). Test results of representative CMU specimens show that unit weight ranged from 1585 to 1743 kg/m3, which was 30-25% lower than that for a normal weight CMU (2300 kg/m3); compressive strengths ranged from 8.4 to 18.7 MPa; water absorption was found to vary between 0.05 to 0.13 g/cm3; and thermal conductivity ranged from 0.27 to 0.41 W/mK. The research findings demonstrate that the use of sedimentary LWA as coarse aggregate in various concrete mixtures could produce high performance lightweight CMU, which comply with the requirements of Chinese National Standards (CNS) standards.

Modeling Corrosion Initiation of Reinforcing Steel in Concrete: Effect of Non-diffusive Coarse Aggregate

The exposure time at which a threshold Cl— concentration, Ct, is initially achieved at the reinforcement depth in concrete, Ti, is an important service life determinant for reinforced concrete structures. However, there is no consensus in the literature of a value for Ct, as reported concentrations extend over one order of magnitude; and it has become increasingly apparent that Ct is distributed even for apparently identical specimens. In this research, models of randomly arranged coarse aggregate (CA) particles in concrete cover were generated for different aggregate types (particle size and size distribution), CA volume fraction, and cover; and, by finite element analysis, the effect of these factors on the time and location along the reinforcement at which Ct is initially achieved was investigated as a function of Ct. The results are discussed within the context of factors that contribute to the reported variability in Ct.

Concrete Shrinkage-Induced Damage Taking into Account Aggregate Restraint

The influence of concrete meso-structure and coarse aggregate size on early age crack is studied. In this study, concrete is regarded as a three-phase material based on shrinkage property. The mortar matrix and interfacial transition zones undergo shrinkage and no spherical coarse aggregate particles are considered to restain the shrinkage by particle interaction. The results show that concrete meso-structure plays an important role in the safety of concrete, different aggregate distribution in concrete leads to different initial cracks and propagation due to shrinkage. With the increase of maximum aggregate size, the value of shrinkage deformation becomes unstable and varies in a wide range; relevant shrinkage cracking becomes easier too.

Portable Image Analysis System for Characterizing Aggregate Morphology

In the past decade, the application of image-based evaluation of particle shape, angularity, and texture has been widely researched to characterize aggregate morphology. Yet the lack of rapid, objective, and quantitative methods for assessment has inhibited its application in the engineering process. However, recent advances in technology have produced pocket computers with as much processing power as some desktop computers. This project takes advantage of these advances to develop an inexpensive portable image analysis system for characterizing aggregate morphology. The system uses an integral pocket computer–high resolution camera but can employ individual components consisting of a digital camera and a laptop or desktop computer. Digital images of coarse aggregate particles are captured with the camera and analyzed within the Matlab software program environment with a macro developed and written for this project to characterize particle morphology with respect to three parameters (shape, angularity, and texture) on the basis of the particle perimeter (outline or edge). For this purpose, several coarse aggregate types from 10 various Virginia sources were analyzed, and the reliability of the image processing was statistically assessed. Asymptotic analysis was performed to determine the number of images needed to obtain a statistically stable value for each aggregate parameter. It was determined that images acquired at close range (2 or 3 in.) were needed to provide sufficient resolution to adequately characterize the aggregates. Also, it was found that statistically valid values for the three aggregate parameters could be obtained from 15 particle images of random but stable orientation, thus making the system efficient in characterizing coarse aggregate morphology.